DE10312370B4 - Connectors for electrodes made of carbon materials - Google Patents

Abstract

Connecting pin for carbon (C) electrodes contains C fibers, surface-activated by oxidation and also having a carbonized coating produced by carbonization of a wax, pitch, natural resin, thermoplastic or thermosetting polymer coating. An independent claim is also included for production of connecting pins of this type.

Description

The Invention relates to connectors for electrodes made of carbon materials. In particular, it relates to connectors that by mixing cokes, pitches and carbon fibers into a mass and Forms of this mass are made and for joining electrodes from carbon materials, in particular graphite electrodes used become.

electrodes from carbon materials, in particular graphite electrodes used in the steel industry in electric arc furnaces. These electrodes consist of individual interconnected cylindrical elements, wherein according to the burning each additional elements are added. The electrodes usually become with connectors (Nipple, English "connecting pins ") with each other mechanically and electrically connected. The connectors have the Form of a double cone (two with the base joined together pyramid stumps) with Threads on the lateral surfaces, the arranged in corresponding centrally in the end faces recesses the cylindrical electrodes fit.

Because of the thermal load, it is necessary to adapt the thermal expansion of the connecting pieces and the electrodes to one another in such a way that no stresses occur which can lead to breakage or other damage to the connection point. In the past, it has been proposed to use carbon fiber reinforced connectors. In the document US-A 4,998,709 ( DE 691 23 778 T2 ) describe graphite connectors reinforced by mesophase pitch derived carbon fibers at a mass fraction of 8 to 20% in the molding composition used in the manufacture. The fibers of mesophase pitch are stabilized oxidatively at elevated temperature and then carbonized in an inert atmosphere at a temperature of 1800 ° C or more. A similar process is described in WO 01/62667, but the fibers also obtained from mesophase pitch have a lower modulus of elasticity and are used in a lower mass fraction (0.5 to 5% in the molding composition). This method leads to a reduction of the coefficient of thermal expansion in the direction of the extrusion or the main axis of the connector.

The stresses normally occurring in fiber reinforced materials are due to the different coefficients of thermal expansion of fibers and matrix. Carbon fibers are practically dimensionally stable in the fiber direction (ie, the coefficient of thermal expansion is slightly negative), whereas for glassy carbon, for example, this coefficient is on the order of 3 × 10 ^-6 K ^-1 .

Opposite this The prior art has the object, while maintaining the low coefficient of thermal expansion of the fiber reinforced connectors also to increase their strength by Improvement of the adhesion of fiber and matrix and the susceptibility to cracking to reduce at thermal stresses.

In the investigations on which the present invention was based, it has now been found that there is a further improvement over the cited known processes (US Pat. US 4998709 , WO 01/62667), if such carbon fibers are used in the molding compound for producing the connecting pieces, the surface of which has been treated prior to mixing into the molding compound and provided with a polymer coating. Connectors according to the invention have a decreased longitudinal (in the direction of the axis of the cylindrical electrode and the connector) thermal expansion coefficient and increased strength. Thanks to these properties, the connecting pieces of the invention not only survive the carbonization temperatures below 1000 ° C. but also a subsequent graphitization treatment at over 3000 ° C.

It It is known to carbon fibers anodically in an electrolyte solution oxidize (J.B. Donnet, R.C. Bansal, Carbon Fibers, Marcel Dekker Inc. New York (1990)). This surface treatment produces oxygen-containing Groups on the fiber surface, usually strong or weakly acidic carboxyl groups and hydroxyl groups, Carbonyl-activated (C-H-acid) C-H groups, as well as basic, pyronic surface groups Bohm, E. Diehl, W. Heck, R. Sappok, Angew Chem 76 (1964), 742; B. R. Puri, in Walker: Chemistry and Physics of Carbon, Vol. 6, Marcel Dekker, New York, (1971), 191).

Also by thermal oxidation without the need for subsequent leaching To remove the electrolytes can be oxygen groups the fiber surface produce. As the oxidation medium, oxygen in different Concentrations, oxygen-halogen mixtures, ozone, carbon dioxide or nitrogen oxides described. A detailed treatment of these topics is found in J. Cziollek ("Studies for influencing the reinforcing behavior of carbon fibers by surface treatment of the fibers and by using a carbon / carbon skeleton as a reinforcing component ", Dissertation Universität Karlsruhe (1983), p. 40 ff).

at carbon fiber Carbon ("CFC") is a reduced Reactivity the fiber surface as a prerequisite for considered a good utilization of the fiber properties. The reduced reactivity is the Matrix the possibility give, from the fiber surface to be able to shrink away. The shrinkage cracks are then repeated by reimpregnation recarbonation steps (Impregnate z. B. with pitch and burning with the exclusion of oxidants) filled. In order to become carbon bond bridges between fiber surface and matrix (J. Cziollek, op. cit.). In addition it is discussed that by the reimpregnation step newly created surface groups the weakened Restore fiber / matrix adhesion (K.H. Geigl: "Studies to surface chemistry of carbon fibers and the development of hollow carbon fibers ", dissertation Universität Karlsruhe (1979)).

in the Contrary to these experimental findings in CFC composites an oxidative surface treatment the fibers in the present invention surprisingly favorable for the properties a composite produced therewith.

object The present invention therefore provides connectors for electrodes made of carbon materials, wherein the connectors are carbon fibers contain their surface oxidatively activated, and in addition a carbonized coating exhibit. This surface coating is the carbonation product of a coating agent (size) selected made of wax, pitch, natural resins, thermoplastic and thermosetting Polymers.

The The invention further relates to a method for the production of connectors which the treated according to the invention Contain fibers.

at the method according to the invention In the first step, the surface of the carbon fibers passes through Oxidation activated, the fibers are then in the second step with a surface coating with a coating agent selected from wax, pitch, natural resins or thermosetting or thermosetting polymers which coated Fibers are optionally at a temperature in the third step from 750 to 1300 ° C treated to carbonize the coating, in the fourth step They are made with coke with a mean particle size in the range from 0.05 to 4 mm, pitch with a softening temperature in the range from 70 ° C up to 150 ° C and optionally further additives mixed and too cylindrical bodies shaped, in the fifth Step, the cylindrical moldings are carbonized and then graphitized, and in the sixth step, the graphitized moldings become the connectors turned off with threads.

there it is preferred that carbon fibers used in the form of fiber cables with 1000 to 60 000 individual filaments be after the third step to short fibers with a medium length be cut from 0.5 to 40 mm.

Further it is preferred that carbon fibers in the form of heavy tow - fiber cables 40 000 to 2 000 000 individual filaments are used after the third process step to short fibers with a middle length of 0.5 to 40 mm can be cut.

Further it is preferred that the Activated carbon fibers are coated in the second step in an aqueous or solventborne Bath containing a dispersion or solution of a coating agent selected made of wax, pitch, natural resins or thermosetting or thermosetting Polymers.

Further it is preferred that the Coated carbon fibers in the third step at a temperature of 900 to 1200 ° C be treated to carbonize the coating.

Further is preferred that in fourth step, a mixture is prepared containing 100 each kg of coke, 10 to 40 kg of a pitch and 0.2 to 20 kg of carbon fibers.

Further is preferred that as further additive 0.1 to 1 kg of an iron oxide pigment with a mean particle size of 0.1 up to 2 μm is added.

The surface coating takes place in particular with polymers that a sufficient Carbon yield at carbonization temperatures of preferred 750 to about 1300 ° C own. Particularly suitable are polyurethane resins, phenolic resins and pitches with a C residue of at least 40% of the mass of the coating composition used.

The Carbonation of the coating agent may be done in one prior to interference in the molding compound temperature treatment step or preferred take place simultaneously with the firing after the green production.

Prefers Carbon fibers are used by carbonating oxidatively stabilized polyacrylonitrile in a known manner available are. On a temperature treatment of the fibers in the range of 1500 ° C to Graphitization temperature (1800 ° C up to 3000 ° C, partly over 3000 ° C) before interference can be dispensed with. The modulus of elasticity of these fibers is preferably 200 to 250 GPa.

The superficial Activation of the carbon fibers takes place by oxidation in one aqueous bath or by an oxygen-enriched gas stream at a temperature from 400 to 600 ° C, the gas stream also fanning the fiber cable is used. In a preferred manner, it is also possible to Carbon fibers electrochemically, ie anodically in aqueous baths oxidize.

Suitable oxidation baths are aqueous solutions of salts of oxidizing acids, such as nitrates, sulfates, chlorates, bromates and iodates, and the said acids themselves; preferred are solutions containing volatile oxidants, preferably the reduction products of these oxidants are also volatile. As volatile here are referred to those substances which are removed without residue or substantially residue-free (with an evaporation residue, which is a maximum of 0.5% of the mass of the treated fibers) during drying of the treated fibers, for example in the air stream or on godets. Particularly preferred oxidizing agents are oxidizing acids such as nitric acid, chloric acid or mixtures containing them with neutral or salt-like inorganic oxidizing agents such as hydrogen peroxide, chromates, permanganates and hypochlorites (KMnO ₄ / H ₂ SO ₄ , K ₂ Cr ₂ O ₇ / H ₂ SO ₄ , HOCl / H ₂ O ₂ / NaOCl). Mixtures of non-oxidizing acids with salt-like oxidizing agents are also suitable, for example the mixtures of hydrochloric acid and chlorates known as euchlorin. In an electrochemical (anodic) oxidation, it is sufficient to produce sufficient conductivity by dissolving acids, bases or salts in water.

To an electrochemical treatment (anodic oxidation) and the Use of oxidizing solutions (especially salt solutions) it is necessary to wash the fiber cables with deionized water, wherein preferably at least two baths arranged one behind the other become. Also the acid-treated Fibers can be washed, the drying step may be omitted can.

The thus activated with oxygen-containing groups carbon fibers are then provided with the above-mentioned surface coating, the dried or only washed fibers passing through an aqueous impregnating bath guided Be the excess coating agent containing solution is squeezed in a known manner and the fiber cable, for example dried on heated godets.

The impregnation is preferably an aqueous preparation said coating agent, for example an aqueous dispersion of waxes, in particular polyolefin waxes based on polyethylene or Polypropylene, as well as montan waxes or by esterification of fatty alcohols with long-chain fatty acids with 12 to 40 carbon atoms synthetically produced waxes. Furthermore, dispersions of polyurethane resins, of (z. B. by grafting with maleic anhydride) activated polyolefins or their copolymers (eg with vinyl alcohol or vinyl acetate) or phenolic resins. It is also possible that activated carbon fibers with in organic solvents dissolved to treat organic compounds, especially those on Base of pitches. Likewise pure pitches in suitable low-viscosity form for coating be used.

The Concentrations of the coating compositions are usually so that one originating from the coating Solid mass fraction on the fiber surface of 0.5 to 30% results. A range of 3 to 15% is preferred. This results in a Mass fraction of carbonized coating on the fibers of preferably 0.2 to 15%.

Prefers are carbon fibers based on (carbonated) polyacrylonitrile used, since it has been shown that these in mixing to the molding compositions according to the invention through the mixing and molding process at the least harmed become. Your elasticity module is usually not as high as that of mesophase pitch based carbon fibers. This means a lower rigidity and therefore a lower shear sensitivity. Both the additional applied Coating as well as the conversion of this coating into one Lead carbon layer to an additional mechanical protection.

It it was observed that the Comminution of the fibers in the interference with the Shear-sensitive HM fibers (high modulus fibers) significantly reduced becomes. The degree of utilization of the amount of fiber used is characterized higher, this leads to additionally to the lower cost of HT fibers ("high tenacity", high-strength fibers) to another Cost advantage.

Likewise By the applied size, the meterability of the short fibers (preferably with a medium length from 0.5 to 40 mm) cut fiber bundles. Uncoated Single filaments with lengths over 2 mm are prone to agglomeration and therefore can not be controlled dosing.

The fiber cables used preferably have from 1000 to 60 000 individual filaments (filaments), Also preferred are multifilament cables based on heavy tow with more than 40,000 filaments and up to 2,000,000 filaments.

It is also possible that the Carbon fibers in the form of parallel strands (so-called "UD tapes"), fabrics, Knitted, crocheted and / or nonwoven fabrics.

The have connectors according to the invention preferably a linear thermal expansion coefficient in the extrusion direction of -0.5 to +0.1 μm / (K · m). The extrusion direction is the direction parallel to the lateral surface of the mostly cylindrically shaped blanks that after carbonation and burning by turning or milling are machined and cut into the required thread become. Perpendicular to the extrusion direction is the linear thermal expansion coefficient preferably from 1.7 to 2.1 μm / (K · m).

Of the Mass fraction of carbon fibers in the connectors is preferred 0, 2 to 10%.

It was in surprising Way found that Connectors, not only made with such carbon fibers through the desired low values for the thermal expansion coefficient, but also an increased Have strength. Both argue for good fiber-matrix adhesion. This can be demonstrated, for example, by the fact that in the Tensile test or in bending test destroyed connectors one electron micrograph of the fracture surface is made and this with a fracture surface of connectors containing fibers without such treatment.

This Findings are illustrated by the microphotographs. It shows

1 an electron micrograph of a fracture surface of a connector (nipple), were used in which carbon fibers according to the invention, and

2 an electron micrograph of a fracture surface of a connector (nipple), were used in which mesophase pitch carbon fibers as reinforcement.

It can be clearly seen from the comparison of the two photographs that in the case of mesophase pitch fibers without the treatment according to the invention ( 2 ) these are simply pulled out of the matrix during a break and leave a hole, whereas in the case of a connector with fibers treated according to the invention by activation and coating, these firmly adhere to the matrix and are not pulled out of the matrix in the event of breakage ( 1 ).

At the fracture surface, in a joint made according to the invention, matrix cracks and cracks of the fibers in the fracture surface are seen. However, there are no holes in the matrix from which the reinforcing fibers were pulled out on failure. The adhesion of the fibers to the matrix is apparently so great that the force required to pull the fibers out of the matrix ("pull-out") is large This is the tensile strength of the fibers. When compared with a connector made in the prior art with mesophase pitch carbon fibers without the treatment of the present invention, the breakout holes of the fibers from the fracture surface are clearly seen.

Surprised is further that how explained in advance, despite the probably better integration of the fibers in the matrix as a result of the surface treatment it to no destruction the fibers come due to internal stresses.

The graphitized bodies made from the compositions of the invention have the following properties:

Connectors off these graphitized bodies to lead in the practical experiment to significantly reduced susceptibility to cracking due to thermal stresses.

The Invention is illustrated by the following examples.

example 1

A fiber cable (7 × 60,000 filaments having a fiber diameter of 7 μm) made of carbonized polyacrylonitrile fibers was subjected to anodic oxidation. For this purpose, the fiber cable was made according to the in US 4,704,196 , Example 3, at a rate of 1 m / min passed through a bath with a length of about 1 m, which contained an aqueous solution of sodium hydroxide solution (5 g in 100 g of the solution), wherein the bath was permanently circulated , In this case, a sinusoidal voltage of 5 V was applied, the current was about 70 A.

Subsequently was The fiber cable in a two-stage wash with deionized Water washed out and squeezed. Then the cable was through a sizing bath containing 10 g of aqueous dispersed polyurethane resin in 100 g of the dispersion with a Wirklength of 0.5 m out, squeezed and over Godets at 120 ° C dried. The fiber cable was cut into staple fibers about 6 mm in length.

Example 2

It was made from a molding material
100 kg of needle coke with a mean particle size of 0.5 mm, 26 kg of coal tar pitch with a softening point (SPM) of 110 ° C., and 3 kg of carbonized PAN-based carbon fibers with a diameter of 7 μm and a mean length of 6 mm, which were oxidized anodically according to Example 1 and provided with a polyurethane coating, and 0.5 kg pigment grade iron oxide (particle size range 0.1 to 2 microns).

The mass was mixed for 0.5 hours in a kneading mixer at 160 ° C, extruded at 120 ° C into a cylindrical strand and after cutting to a length of about 3000 mm at 800 ° C for 500 hours the burned. The fired cylindrical carbon bodies were then impregnated three times with an impregnating pitch (SPM 80 ° C.) and postbaked at 800 ° C. The impregnated and baked carbon bodies were graphitized in a conventional manner at about 3000 ° C.

The following values were measured on the cylindrical graphite bodies of 305 mm diameter and 2300 mm length (for comparison, a corresponding mixture without added fiber was prepared, the measured values on the graphite bodies produced therefrom are given in parentheses):

Claims (18)

Carbon material electrode connectors, characterized in that the connectors comprise carbon fibers whose surface is oxidatively activated and which additionally have a carbonized coating, the carbonized coating being the carbonation product of a coating agent selected from wax, pitch, natural resins , thermoplastic and thermosetting polymers. connectors according to claim 1, characterized in that the carbon fibers a modulus of elasticity from 200 to 250 GPa. connectors according to claim 1, characterized in that it is a linear thermal Expansion coefficients of -0.5 to +0.1 μm / (K · m) in the direction parallel to the lateral surface and 1.7 to 2.1 μm / (K · m) in have the plane perpendicular thereto. connectors according to claim 1, characterized in that the carbon fibers a medium length from 0.5 to 40 mm. connectors according to claim 1, characterized in that the mass fraction of carbon fibers in the connectors from 0.2 to 10%. connectors according to claim 1, characterized in that the mass fraction of the carbonized Coating on the carbon fibers, based on the mass of Carbon fibers, 0.2 to 15%. connectors according to claim 1, characterized in that the carbon fibers such based on polyacrylonitrile. connectors according to claim 1, characterized in that the carbon fibers in the form of parallel threads, Woven, laid, knitted, crocheted and / or nonwoven fabrics. Process for the preparation of carbon fibers containing connectors for electrodes of carbon materials, characterized in that in the first Step the surface the carbon fibers is activated by oxidation that the fibers subsequently in the second step with a surface coating with a Coating agent selected made of wax, pitch, natural resins or thermosetting or thermosetting Polymers are provided, the coated fibers optionally in the third step at a temperature of 750 to 1300 ° C for carbonation the coating will be treated with coke in the fourth step a mean particle size in the range from 0.05 to 4 mm, pitch with a softening temperature in the range from 70 ° C up to 150 ° C and optionally further additives mixed and too cylindrical bodies be formed, in the fifth Step the cylindrical shaped body carbonized and then be graphitized, and in the sixth step, the graphitized moldings the connectors be turned off with threads. Method according to claim 9, characterized in that that carbon fibers used in the form of fiber cables with 1000 to 60 000 individual filaments be after the third step to short fibers with a medium length be cut from 0.5 to 40 mm. Method according to claim 9, characterized in that that carbon fibers in the form of heavy tow - fiber cables 40 000 to 2 000 000 individual filaments are used after the third process step to short fibers with a middle length of 0.5 to 40 mm can be cut. Method according to claim 9, characterized in that that the Carbon fibers in an aqueous medium containing an oxidizing agent to be activated. Method according to claim 9, characterized in that that the Carbon fibers in an aqueous bath activated by anodic oxidation. Method according to claim 9, characterized in that that the Carbon fibers in a gas stream containing an oxidant to be activated. Method according to claim 9, characterized in that that the Activated carbon fibers are coated in the second step in an aqueous or solventborne Bath containing a dispersion or solution of a coating agent selected made of wax, pitch, natural resins or thermosetting or thermosetting Polymers. Method according to claim 9, characterized in that that the coated carbon fibers in the third step at a temperature from 900 to 1200 ° C be treated to carbonize the coating. Method according to claim 9, characterized in that that in the fourth step, a mixture is prepared containing 100 each kg of coke, 10 to 40 kg of a pitch and 0.2 to 20 kg of carbon fibers. Method according to claim 17, characterized in that that as further additive 0.1 to 1 kg of an iron oxide pigment with a mean particle size of 0.1 up to 2 μm is added.